The coadsorption behavior of Cu(II) and ciprofloxacin (Cip), a zwitterionic fluoroquinolone (FQ) antibiotic,
to goethite surfaces was characterized by means of batch adsorption experiments, attenuated total reflectance
Fourier transform infrared (ATR–FTIR) spectroscopy, and extended X-ray absorption fine structure
(EXAFS) spectroscopy. The collective quantitative and spectroscopic results indicate that Cip was
adsorbed onto goethite surfaces through a tridentate complex involving the bidentate inner-sphere
coordination of the deprotonated carboxylate group and H-bonding through the adjacent carbonyl group
on the quinoline ring. In contrast, in the presence of Cu(II), Cip adsorption is enhanced around pH 6.
Spectroscopic results showed that a goethite–Cu–Cip ternary surface complex is formed through the
bidentate coordination of Cu(II) with the oxygen from the deprotonated carboxylate group and the adjacent
carbonyl oxygen. A charge distribution (CD) surface complexation model constrained by spectroscopic
observations was developed to describe macroscopic adsorption trends. The model well
described the adsorption of Cip and Cu(II) under various conditions with one set of parameters. These
findings will help quantitatively predict the adsorption behavior of Cip and Cu(II) onto ferric oxides